1. Field of the Invention
This invention relates to a method for collecting a reversely subliming substance from a gas containing the substance and an apparatus for the collection.
2. Description of Related Art
Various reports have been heretofore published concerning a method including synthesizing pyromellitic anhydride by the reaction of catalytic gas phase oxidation from durene, for example, as a raw material and collecting it from a gas containing the produced. Several methods utilizing a cooled wall surface have been available as shown below to obtain crystalline pyromellitic anhydride of a large size.
U.S. Pat. No. 4,252,545 discloses a method which includes causing a pyromellitic anhydride-containing gas to flow at a flow velocity in the range of 1-3 m/sec parallelly to a cooled surface maintained at an equilibrated temperature and recovering consequently formed crystals by the technique of fractional reverse sublimation. This method includes two versions, one causing the gas to flow down the interiors of a plural of tubes and the other causing the gas to flow to the heat-exchange surface of a plate curer. Basically, the method is intended to avoid deposition of crystals on the cooled wall surface to the fullest possible extent. The crystals thus deposited are so fast thereon to be periodically peeled by washing or mechanically. The method renders the operation of the relevant apparatus difficult.
U.S. Pat. No. 3,693,707 discloses an apparatus for precipitating crystals by reverse sublimation from a pyromellitic anhydride-containing gas by the use of fin tubes. This apparatus inevitably proves to be a very commercially expensive because the crystals once adhered to complicated surfaces such as the fin tubes are not easily peeled and recovered.
JP-B-61-121 discloses a method which includes exposing a pyromellitic anhydride-containing gas to reverse sublimation thereby inducing deposition and growth of crystals of the acid on a cooled wall surface, then elevating the temperature of the wall surface to a level exceeding the subliming temperature thereby effecting removal of adhered crystals by sublimation, and causing the residual grown crystals to separate from the wall surface and fall down, and an apparatus therefor. This method, however, entails the problem that the apparatus is complicated and that the loss in collection is aggravated because of the sublimation removal of the crystals.
JP-A-4-131,101 discloses a method which includes causing an air current containing a sublime compound to entrain wear-resisting particles thereby inducing precipitation of crystals to the cooled surface of a condenser and, at the same time, allowing the wear-resistant particles to collide against the crystals and peel them in a powdery form. This method, however, entails the problem that the product is only a powdery form and that the apparatus is subjected to wear by the collision of the particles.
JP-A-10-265,474 discloses a method which includes inducing precipitation of crystals on the cooled surface of a vertical tubular condenser and then elevating the temperature of the surface thereby causing the crystals to peel by subliming pressure. JP-A-10-279,522 discloses a method which includes inducing precipitation of crystals on a cooled surface and then lowering the temperature of the surface thereby causing the crystals to peel by virtue of the difference in shrinkage between the surface and the crystals. In the specification of this invention, no actual mode of embodiment is mentioned anywhere about a shell-and-tube type collector that is effective in commercially operating the method.
Thus, none of the methods is known sufficiently for commercial production of a reversely subliming substance of high purity and large size such as pyromellitic anhydride directly from a gas phase.
An object of the present invention is to provide a method for the recovery by fractional reverse sublimation a reversely subliming substance such as pyromellitic anhydride from a gas containing the substance in which highly pure, highly collected crystals are expelled out of the collection system, and an apparatus therefor.
The object of this invention is accomplished by a method for collecting a reversely subliming substance from a gas containing the substance by the use of a shell-and-tube type collector capable of effecting adhesion and collection of the substance on the inside surfaces of cooled collection tubes in which the pressure loss at which the gas passing through the cooled tubes is adjusted in the range of 0.05 to 50,000 Pa.
The object of this invention is also accomplished by a method for collecting a reversely subliming substance from a gas containing the substance by the use of a shell-and-tube type collector capable of effecting adhesion and collection of the substance on the inside surfaces of cooled collection tubes in which the cooled tubes have a common, empty space formed in the lower part of the collector for gas-introducing tubes and the gas is introduced within the empty space by adopting at least one of the following means:
(1) a gas-introducing tube with a perforated plate at the outlet part thereof,
(2) a gas-introducing tube the outlet direction of which has an angle larger than 90xc2x0 relative to the directions in which the gas passes through the cooled tubes, and
(3) two or more gas-introducing tubes.
The object of the invention is further accomplished by a shell-and-tube type apparatus for adhering and collecting a reversely subliming substance from a gas containing the substance in which the outlet parts of cooled tubes formed in the shell-and-tube type apparatus are each provided with baffle plates.
According to the method of the invention, it is possible to level the amount of the gas containing the reversely subliming substance which passes through the individual cooled tubes and increase the collection coefficient of the substance by adjusting the resistance at a prescribed pressure loss at which the gas passes through the cooled tubes retained in the shell-and-tube type collector.
According to the method of the invention, the amount of the gas containing the reversely subliming substance which passes through the individual cooled tubes retained in the shell-and-tube type collector can be leveled and the collection coefficient of the substance can be increased since the gas can be efficiently dispersed during the collection thereof by the use of a shell-and-tube type collector.
According to the collector of the invention, it is possible to level the amount of the gas containing the reversely subliming substance which passes through the individual cooled tubes and increase the collection coefficient of the substance by introducing the gas into the collector with a gas-introducing tube which excels in the ability to disperse the gas and disposing a baffle plate to each at the outlet parts of the cooled tubes retained in the collector thereby controlling the resistance of the cooled tubes.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.